In order to work and to get taxed properly in New Zealand, you need to obtain a tax number from the New Zealand Government. You can apply for this when you arrive in NZ. Don't worry, it isn't as hard as it sounds:
1) Download the Tax Number Application Form (IR742) from the Inland Revenue website (www.ird.govt.nz) For a direct link to the application form click here.
2) Print out and complete the application form. You will need copies of the the following documentation:
Photographic identification – This is most typically a copy of your passport information page – the one where your photo is.
Proof of permanent address - Proof of your current or last address (can be from your home Country) in the form of an official bill or bank/credit card statement for example or a copy of your driver's licence (where there is an address shown).
Taxpayer’s identification number - If you have previously paid tax in any country you will need to show proof of this. This will be in the form of a taxpayer identification number (TIN) which goes by different names in different countries. New Zealand's TIN is the IRD number.
Work visa - A copy of your work visa or any written job offer which is used to prove the purpose of your stay in New Zealand.
NZ Bank Account - Evidence that you have set up a fully functional New Zealand bank account such as a recent statement
3) Now take the completed form and photocopies of your supporting documents to either a New Zealand Post Shop, Automobile Association offices or post to Inland Revenue, PO BOX 39050, Wellington Mail Centre 5045, New Zealand. If you're outside New Zealand you can email them to email@example.com.
4) Your IRD number confirmation will be sent to the postal address you supplied (you can get an NZ postal address with our mail service) within 8-10 days or will be emailed/texted to you if you select this option. You can then give this to your employer when you start working. For more info on income tax see the NZ Inland Revenue website.